My iPhone 4S developed the same issue the past few days. Tried resetting, shutting the phone off, flipping the vibrate button on and off but nothing works.
No ability to use the speaker phone, incoming calls do not have any sound, no text sounds, alarm sounds, or keyboard sounds, etc. Can talk on the phone and hear others talking to me but that's about it. Phone has not been dropped, not damaged and not exposed to water. Going back to swap this phone out again...
Maybe this time they won't give me another refurbished/remanufactured phone on a warranty exchange. Speaking from experience, they are NOT the same as new iPhones. This one has been used for less than 100 hours of talk time and its on the fritz. The last one never worked right but I tried troubleshooting the dropped calls/poor sound quality with them for two weeks before swapping it out for this one.
Hope they are working to fix these issues...never had cell phone issues like this before!